1. Field of the Invention
Embodiments of the invention relate to downhole steam generators.
2. Description of the Related Art
There are extensive viscous hydrocarbon reservoirs throughout the world. These reservoirs contain a very viscous hydrocarbon, often called “bitumen,” “tar,” “heavy oil,” or “ultra heavy oil,” (collectively referred to herein as “heavy oil”) which typically has viscosities in the range from 3,000 to over 1,000,000 centipoise. The high viscosity makes it difficult and expensive to recover the hydrocarbon.
Each oil reservoir is unique and responds differently to the variety of methods employed to recover the hydrocarbons therein. Generally, heating the heavy oil in situ to lower the viscosity has been employed. Normally reservoirs as viscous as these would be produced with methods such as cyclic steam stimulation (CSS), steam drive (Drive), and steam assisted gravity drainage (SAGD), where steam is injected from the surface into the reservoir to heat the oil and reduce its viscosity enough for production. However, some of these viscous hydrocarbon reservoirs are located under a permafrost layer that may extend as deep as 1800 feet. Steam cannot be injected though the permafrost layer because the heat could potentially expand the permafrost, causing wellbore stability issues and significant environmental problems with melting permafrost.
Additionally, the current methods of producing heavy oil reservoirs face other limitations. One such problem is wellbore heat loss of the steam, as the steams travels from the surface to the reservoir. This problem is worsened as the depth of the reservoir increases. Similarly, the quality of steam available for injection into the reservoir also decreases with increasing depth, and the steam quality available downhole at the point of injection is much lower than that generated at the surface. This situation lowers the energy efficiency of the oil recovery process.
To address the shortcomings of injecting steam from the surface, the use of downhole steam generators (DHSG) has been employed. DHSGs provide the ability to heat steam downhole, prior to injection into the reservoir. DHSGs, however, also present numerous challenges, including excessive temperatures, corrosion issues, and combustion instabilities. These challenges often result in material failures and thermal instabilities and inefficiencies.
Therefore, there is a continuous need for new and improved downhole steam generator designs.